Thanks, @Önder.Sönmez ! 

@Raju.G I’ll try to go through your questions one by one.

1. Unfortunately, different types of paint and different colors of paint all have unique scattering properties, so there isn’t an existing model that would be a fair evaluation of your surface. You have a few options to model the surface scatter behavior (which I mentioned in https://community.zemax.com/questions/829), but ultimately OpticStudio relies upon you knowing the scatter distribution of your surface and applying that to the model. If you don’t know the scatter distribution of your paint, you could try to measure it (OpticStudio won’t help here, you’d need some lab equipment) or estimate it. You could use any of the existing scatter models for your estimate or create your own scatter model.
2. Yes, it’s possible to set the wavelength of your system to 2.7 microns and simulate the scattering from your paint using this wavelength, but again as mentioned in #1, this fundamentally relies upon your knowledge of the scatter distribution of your material for a 2.7 micron wavelength, and you inputting that information into your OpticStudio system model. If you were hoping that OpticStudio would be able to tell you the scatter distribution in this case, you wouldn’t be the first OpticStudio user to be disappointed that this is a required input for OpticStudio, not an output.
3. Yes, that sounds like a reasonable set-up. 
4. Yes, the easiest way to do this would be to use the Ref Obj column on the Source Ray object and set it to refer to your rectangular volume. Then you can use the Tilt About X field to tilt by 45 degrees. I think you’ll find this article helpful in understanding this concept and how it applies to the model. https://support.zemax.com/hc/en-us/articles/1500005487701-How-to-use-the-Modify-Reference-Object-tool